The proposed study is designed to evaluate the efficacy of micronuclei (morphologic markers of biochemical lesions) for monitoring human exposure to environmental mutagens. The experimental model is based upon the circulating lymphocyte because it is readily and repeatedly available in large numbers, circulates in all tissues, proliferates easily in culture, transports genotoxic agents, is relatively long lived and can thus serve to integrate exposure over time. The basic hypothesis is that the frequency of nuclear DNA lesions, expressed as micronuclei, will increase in proportion to the magnitude of the genotoxic insult from ether physical or chemical stimuli. This study will investigate the quantitative relationship between precisely characterized human exposure to ionizing radiation and the measurement of micronuclei (DNA damage) in lymphocytes obtained from the exposed subject. Phase I studies will involve further laboratory development, testing and validation under conditions of in vitro exposure to both chemicals and radiation. Human lymphocytes will be isolated, cultured, harvested by cytocentrifugation, stained and analyzed for micronuclei using manual and computer-enhanced video microscopy. An innovative component of the study will be the development of an automated cell image analysis program to rapidly scan homogeneous cell monolayers to detect and quantify the frequency of micronuclei in large populations of stimulated lymphocytes. Prospects for enhancing the sensitivity and thus application of the assay are excellent and will be significantly increased by utilizing 1) monoclonal antibodies to identify the proliferating (first generation) cell population and 2) use of computer controlled quantitative cell image analysis methods to analyze larger samples of cells per individual and larger numbers of individuals per cohort. Phase II will then extend the investigation to involve selected human subjects undergoing specific diagnostic and radiotherapy procedures to further evaluate the method and define its limits of sensitivity under actual conditions of in vivo exposure. The design of the study will also involve sequential sampling of exposed subjects and will thus provide important data on the temporal persistence of the biochemical marker. The persistence of such lesions is an important consideration in future epidemiologic studies. Our encouraging preliminary studies show that DNA lesions giving rise to micronucleated lymphocytes are sensitive to genotoxic chemicals at concentrations in the parts per billion range and radiation exposures in the 10-20 rad range. If Phases I and II are successful, further investigation will be proposed using prospective epidemiologic studies to evaluate the predictive power of the marker in identifying high risk and susceptible individuals. Altogether, the method described in this application may have value as a biological dosimeter to detect subtle chronic exposures to environmental agents that may be associated with adverse health effects.